@uberwaltz, the reason could be as simple as the tubes in a component pick up any mechanical or acoustical sound/noise and introduce it into the circuit. Relating to Almarg's comments, all tubes are inherently microphonic to some degree however small. I don't mean in the sense a tube has become "microphonic" due to damage, poor design, etc. But the fact that if you touch a tube, you will still hear the sound thru the system.
This type of noise/vibration would not be present in a SS component.
This type of noise/vibration would not be present in a SS component.